The invention relates to optical isolators, and in particular to bi-directional optical isolators having a first comb filter response in a first direction through the device, which is different from a second comb filter response in a second opposite direction through the device.
An optical isolator is a device intended to prevent return reflections along a transmission path. Isolators are commonly used optical components that can be used, for example, to prevent reflections in an optical fiber from interfering with the transfer of signals. Current isolators are uni-directional devices that allow optical signals to pass in one direction and not to pass in the opposite direction.
To provide a functional bi-directional optical system, uni-directional isolators are used for transmission in each direction. Requiring isolator sets for each direction increases the cost and complexity of an optical network in which bi-directional isolation is required. Therefore, it is desirable to provide a bi-directional optical isolator.
One prior art polarization independent optical isolator is described in U.S. Pat. No. 5,033,830 entitled Polarization Independent Optical Isolator, issued Jul. 23, 1991 in the name of Jameson. Jameson describes an isolator having a single birefringent plate, a pair of stacked reciprocal rotators, a Faraday rotator, and a reflector positioned in tandem adjacent to the birefringent plate. In a forward (transmitting) direction, a light wave signal exiting an optical fiber is split into a pair of orthogonal rays by the birefringent plate. The orthogonal rays then pass through a first reciprocal rotator and the Faraday rotator, which provides 22.5xc2x0 of rotation. The rotated rays are then redirected by the reflector back though the Faraday rotator. After passing through the second reciprocal rotator, the orthogonal rays re-enter the same birefringent plate where they are recombined and launched in an output fiber. Since a Faraday rotator is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through the birefringent plate such that neither will intercept the input fiber.
An isolated optical coupler is disclosed in U.S. Pat. No. 5,082,343 issued Jan. 21, 1992 in the name of Coult et al. The coupler described in the patent is comprised of a pair of lenses having a wavelength selective device and an isolator disposed therebetween. Another optical isolator, which attempts to improve upon Coult""s design, is described in U.S. Pat. No. 5,594,821 issued in the name of Yihao Cheng. Yet another optical isolator is described in U.S. Pat. No. 5,267,078 issued in the name of Shiraishi et al.
Although these prior art devices appear to provide their intended function of isolating in a unidirectional manner, substantially preventing light from propagating in a backward direction, while only allowing light to pass in a forward direction, it is an object of the present invention to provide a wavelength dependent isolator that in one mode of operation allows a first group of periodic wavelengths to pass in a first direction from a first port to a second port, while substantially preventing a second group of periodic wavelengths to pass, and simultaneously allowing a second group of wavelengths to pass in a second direction from the second port to the first port while substantially preventing the first group of wavelengths from passing in the same direction.
It is another object of this invention to provide a two-port isolator having a comb filter response that is different in one direction than the other.
Unlike prior art optical isolators generally used to allow signals to propagate in a forward direction but not in a backward direction, the isolator in accordance with an embodiment of this invention allows propagation of signals through the isolator in both directions from a first to a second port and vice versa, wherein propagation is wavelength dependent and mutually exclusive with respect to wavelengths that are able to pass in each direction.
Accordingly, the present invention relates to a bi-directional isolator comprising:
a first port for launching a first optical signal comprising at least one wavelength channel from a first set of wavelength channels, and for outputting a second optical signal comprising at least one wavelength channel from a second set of wavelength channels, independent of the first set of wavelength channels;
a second port for launching the second optical signal, and for outputting the first optical signal;
first routing means for directing the first optical signal from the first port to the second port, while preventing signals comprising at least one wavelength channel from the second set of wavelength channels from passing thereto; and
second routing means for directing the second optical signal from the second port to the first port, while preventing signals comprising at least one wavelength channel from the second set of wavelength channels from passing thereto.